The present invention relates to a mobile communication network system and, more particularly, to a spread spectrum communication system in which predetermined pilot symbols are time-multiplexed into a radio signal in each of physical channels.
Spread spectrum communication systems have a good interference canceling function to minimize or even eliminate blocking and fading. Thus, a radio signal is received with excellent characteristics on a multi-path environment. Such systems have been examined in recent years on their applicability to mobile communications. Since the spread spectrum communication systems have high spread frequency, the multi-path propagation paths are distinguished from each other and a RAKE reception system can be applied. The RAKE reception system is a scheme that provides a path diversity effect. More specifically, RAKE reception system separates the multi-path propagation paths and estimates propagation path characteristic of each path. Then, compensation operation of the propagation path characteristics is carried out to each of the reception signals of each of the paths. Maximum-ratio combining or common-mode combining of compensated propagation path characteristics is carried out. The RAKE reception system comprises a finger unit which carries out cancel process of the propagation path characteristics by carrying out estimation of the channel characteristics of the multi-path propagation paths and carrying out multiplication by the use of complex conjugate of the estimated channel characteristics. (Andrew J. Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison-Wesley Publishing Company, page 89, 1995 (hereinafter, referred to as an article 1). The finger unit comprises a correlation unit to de-spread the spectrum spread codes. For effective RAKE combining, the characteristics of the channel estimation in the finger unit are very important. Some systems insert predetermined pilot symbols periodically by every certain number of slots for each physical channel. For such systems, various approaches have been proposed that provide channel estimation or signal-to-interference (SIR) ratio estimation for reception signals using pilot symbols for a reception physical channel. (See, for example, H. Ando and M. Sawahashi, Space Diversity Characteristics of Channel Estimation RAKE using DS-CDMA multi-pilot blocks, 1997 IEICE General Conference, B-5-13 (hereinafter, referred to as an article 2); Azuma, Ohguchi, and Ohno, Characteristics of Interpolation-type Synchronous Detection RAKE for DS/CDMA, Technical Report of IEICE. RCS94-98, pages 57-62, 1994 (hereinafter, referred to as an article 3); and Kiyoo, Y. Okumura, and T. Doi, Examination about SIR Measurement Method in Adaptive Power Control for DS-CDMA, 1996 IEICE Communications Society Conference, B-330 (hereinafter, referred to as an article 4). The methods in the articles 2 and 4 provide channel estimation for a certain slot using a pilot symbol for the adjacent slot. On the other hand, the article 3 uses data contained in a certain physical channel based on a feedback decision technique for more accurate channel estimation. It is noted both methods basically use only the pilot symbol interpolated in the reception physical channel.
The above-mentioned conventional methods use the reception signals on the certain physical channel and thus have significant limitations to improvement of the characteristics on the RAKE reception and SIR estimation. In the heart of the city having towering buildings, multi-paths are often used. In this case, paths are established and released frequently. Thus, the conventional method provides a smaller Eb/No or Eb/Io ratio where Eb is a signal electric power, No is a noise electric power, and Io is interference electric power. In addition, the methods provide a smaller effect of reducing time-averaging noise level in association with the time fluctuation of the noise on the paths. This means that the small Eb/No ratio deteriorates accuracy of temporal data decision if used as in the article 3. Characteristics can thus be improved only to a limited extent. On the other hand, the techniques as in the articles 2 and 4 have limitation to the level of accuracy improvement because only a small number of slots are available due to the time fluctuation of the propagation characteristics when the symbols used are for two or more slots.